Interview questions and answers - Instrumentation /Automation /Process control engineers

  1. What does PID mean? (Be specific, don’t just define the Three Letter Acronym)
    Acronym: P=Proportion or gain applied to error to assist in overcoming the deviation.

I= Proportion or gain over time applied to error to assist in returning to setpoint.

D= The amount of gain over time needed to overcome large upsets or deviation from setpoint

2.PID is normally used for three-mode control. What does this mean?

This means that all 3 modes, proportional, Integral and Derivitive are used in tuning a control loop for optimum results.

3.What is a cascade loop?

Cascade loop is: Where the output of one controller is fed to the setpoint of another controller optimum control.

4.What is meant by the term setpoint?

The setpoint is: the desirable controlled value

5.What is a single loop controller

Single loop means: Has one input, controller and output in a closed loop.

  1. Discuss the interrelationship between SP, PV and Output.?

SP is the: Is the desired process variable

PV is the: Is the actual process variable

Output is the: Is the applied variable to bring the PV equal to the SP

7.What is a marshalling panel?

Marshalling Panels provide: Coordinated location for field terminations in rack room to then connect to process equipment.

  1. In process control, what is the function of a transmitter?

A transmitter: a measured variable from a primary element to provide a scalable PV

9.What is meant by the term 4 to 20?

This is a: Is a common medium in milliamps to transfer analog values

10.What is an RTD?

That is a precision temperature primary element using resistance.

  1. In process control, what do the abbreviations, AI, AO, DI and DO typically stand for?

AI: analog input
AO: analog output
DI: digital input
DO: digital output

12.What is a closed-loop system?

A type of control system that: has feedback from the PV changed by the manipulated variable.

13.What is a thermocouple and how does it work?

A device for: measuring temperature. a primary element for temperature. It works on the seebeck effect principle, reading mV between hot and cold junctions.


Sir thermocouple works on Sea back effect sir. But u said it’s works on biametilic.